Most of the nitrated compounds used by the Army are in the form of explosives and propellants. Current nitration methods often result in low yields of desired products and complex mixtures of reaction products from side reactions. Complex mixtures of reaction products in propellants can produce undesired results.
Traditionally, nitration of paraffins in the thermally-induced methods is difficult to accomplish. High temperature (200.degree.-400.degree. C.) and high pressure (8-12 atmospheres) reactors are used with some success in the commercial production of nitroalkanes. However, new methods are continually sought which will increase the currently low yields of nitroparaffins and minimize the troublesome side reactions. Efficient control over which nitroalkanes are produced (selective nitration) in these processes is also desirable.
Recently, Umstead, et al. J. Quant Elect. QE-16, 1227 (1980), reported the photonitration of isobutane by nitrogen dioxide (NO.sub.2) using an argon-ion laser as the excitation source. The NO.sub.2 absorbs the radiation and then according to the kinetic modeling Unstead et al, Applied Physic B38, 219 1985) the vibronically excited NO.sub.2 (NO.sub.2 *.sup.+) achieves the direct abstraction of a hydrogen atom from the isobutane. The resulting free radical reacts primarily with NO.sub.2 to form 2-methyl-2-nitropropane. However, the yield of 2-methyl-2-nitropropane based on isobutane was low (about 2.6%) and product fragmentation was reported to be significant.
The thermally-induced reaction between alkanes and NO.sub.2 is described as proceeding via a free radical mechanism. The primary reaction steps are thought to be: EQU RH+NO.sub.2 .fwdarw.R.sup..multidot. HNO.sub.2 .fwdarw.R.sup..multidot. +.sup..multidot. OH+NO (1)
and EQU R.sup..multidot. +.sup..multidot. NO.sub.2 .fwdarw.RNO.sub.2 .multidot.(2)
In the thermally-induced nitration of hydrocarbons, RH, using NO.sub.2, an important side reaction is EQU R.sup..multidot. +.sup..multidot. ONO.fwdarw.RONO (3)
in which an unstable alkylnitrite, RONO, is formed. The alkylnitrite decomposes as follows: EQU RONO.fwdarw.RO.sup..multidot. +NO (4)
although at conditions usually employed for nitrating hydrocarbons with NO.sub.2, some of the alkylnitrite might not decompose. The alkoxy radical, RO, produced in reaction (4), may undergo any of a number of reactions, including the following, using propyl radicals as examples: ##STR1##
The continued oxidation of aldehydes, ketones, alcohols, etc. produces acids, oxides of carbons, water, etc. The alkyl radicals formed in reactions (5), (6), and (8) could react with NO.sub.2 to form lower molecular weight nitroalkanes. Alkyl radicals can also decompose directly to form lower molecular weight alkyl radicals which could react with NO.sub.2 to form shorter chain nitroalkanes.
A method to achieve nitration of hydrocarbons which produces less undesirable compounds in side reactions as illustrated by the above equations for thermally-induced reaction is highly desirable, particularly to meet both explosive and propellant requirements of the Army.
Therefore, an object of this invention is to provide a method for laser-induced nitrations of hydrocarbons wherein the nitration reactions are driven toward specific end products required for propellants with the chemical composition necessary to burn exactly with high energy production while having a minimum of side products which create undesirable smoke.
Another object of the invention is to provide a method for laser-induced nitrations of hydrocarbons to selectively nitrate these hydrocarbons to secondary nitrohydrocarbons having the same chain length as the reacting hydrocarbon.